1. Field of the Invention
The present invention relates to a storage medium reproducing apparatus, a storage medium reproducing method, and a computer program product for reading information from a storage medium.
2. Description of the Related Art
In recent years, semiconductor memories such as flash memories for storing information depending on charge quantities have been widely known. Also, multi-value memory techniques by which a plurality of threshold values are set for charge quantities have been developed so as to store information of a plurality of bits.
In such a semiconductor memory, charges are released with time. Therefore, when the quantity of released charges exceeds a threshold value, an error is generated at the time of information reading. Especially, in a multi-value memory having threshold values at narrow intervals, the probability of generating an error is high.
Meanwhile, so as to maintain the reliability of stored information, error correction codes are allotted to memory cell groups each containing a certain number of memory elements, and errors in information are corrected using the error correction codes according to a method disclosed in Japanese Patent Application Laid-Open No. H11-154394, for example.
However, a memory element that has the charge quantity exceeding the threshold value due to discharge constantly generates an error at the time of reading. Therefore, according to the conventional error correcting method, the same error correcting operation needs to be performed every time reading is performed, resulting in a large calculation amount in each correcting operation.
In addition to the memory elements that constantly generate errors due to discharging, there might be memory elements that newly generate errors. So as to correct the errors of such a large number of memory elements, a large number of correction codes are required. This results in an even larger calculation amount in each correcting operation.